1. Field of the Invention:
This invention relates to an angular position sensing and control system and more particularly to such a position sensing system for monitoring the orientation of a structural body relative to a gravity established local horizontal reference for stabilizing a structural body mounted platform relative to the local horizontal reference.
2. Description of the Prior Art:
Difficulty has been experienced in the past in monitoring the angular orientation of a structural body with reference to earth's coordinates and in controlling a platform or other movable part mounted within the body to maintain a predetermined orientation referenced to earth coordinates while the structural body is experiencing three dimensional angular and translational motion. The usual approach for monitoring orientation or stabilizing a platform involves the use of a vertical gyro with its attendant disadvantages stemming from its relatively short operating life and deterioriating accuracy as the spinning rotor and gimbal bearings deteriorate with extended use. The wear out life time of the vertical gyro represents a serious and significant problem of periodic replacement as well as a danger of loss of data or erroneous platform position control due to untimely and unexpected gyro failure modes.
Attempts have heretofore been made to control the attitude of one part of a machine with respect to some reference. For example, attempts have been made to control the attitude of a blade of a motor grader by utilizing electrolytic potentiometers or linear accelerometers to provide a gravity-referenced local horizontal. This served to sense liinear acceleration only, and it has been found that such a system has numerous disadvantages. For example, lateral movements of the entire road grader produced a command signal to the blade tilt control which made the hydraulic system move one end of the blade up and down, which in turn shook the machine from side to side, so that in fact the motor grader was oscillatory at the natural frequency of the grader. For this and other reasons, it has been found necessary to limit the speed of response of the attitude control system and to operate such modified graders at very low speeds, on the order of one mile per hour.
At the present time, road graders are being operated with the blade controlled manually by the operators. This requires extreme skill on the part of the operators, because it is necessary for them to control blade elevation and cross-grade angle simultaneously. Thus, the operator has to pay very close attention to the position of both the right and left ends of the blade, which presents great difficulty because the operator is in the operator's station elevated a substantial distance over the blade.
There is a need for an angular position sensing and control system which is not subjected to the stringent structural requirements placed on component parts associated with the high angular velocity spinning rotor of a gyro, and which does not require the ultra low torque bearing supports associated with gyro gimbals. A new apparatus and method is needed which may be utilized for controlling the angular movement about an axis of one part of a structural body or machine relative to a gravity-referenced local horizontal plane.